Continuous technology improvements in the semiconductor industry are consistently increasing the achievable logical gates density. This trend first formulated by Moore, and now well referred to as Moore's law, drives a higher integration of logic into standard silicon. Today, complete wireless modem subsystems can be integrated together with complex audio/video subsystems, application processors and Random Access Memory (RAM) onto one silicon die. As digital logic is the first to take advantage of new technology processes, digital logic typically is the first that can be ported to a new process with which an even higher integration can be achieved. Analog options for a new technology, especially those needed for building radio frequency (RF) components, typically follow later. Thus, it can be very desirable to divide a wireless modem into at least two components, a RF transceiver and a digital Baseband (BB), where the BB can be ported to a new technology as the option becomes possible. A digital interface between the RF transceiver and the BB can be advantageous in such a case to ease integration, particularly when the digital interface is (more or less) technology independent. A digital RF-BB interface further possesses the advantage that the Analog-to-Digital and Digital-to-Analog Converters (ADCs/DACs) reside on the RF side, thereby reducing the need for analog components on the digital die even further.
Currently, the Mobile Industry Processor Interface Alliance (MIPI) group has standardized a serial digital RF-BB interface for cellular phones specified by the DigRF v3 standard. The standardization group is now focusing on other wireless broadband standards, such as the Worldwide Interoperability for Microwave Access (WiMAX™) technology and the Long Term Evolution (LTE) standard under development, also called the DigRF v4 or 4G or 3.9G standard. The digital RF-BB interface allows for transmitting and receiving digitized in-phase (I) and quadrature (Q) samples multiplexed with control commands, respectively towards and from the transceiver. The above DigRF standards also include so-called Time Accurate Strobe (TAS) messages, which are used by the BB for accurate timing of the RF commands. While more digital functions are integrated on the RF side, a simple TAS may not be sufficient due to the variable delays and the extensive sets of commands with various timing moments in the RF transceiver.
The DigRF v4 standard is optimized for power consumption, which is achieved by minimizing the number of interface lines and using asynchronous communication with high-speed data bursts. This is in contrast to previous (prior art) analog and digital interfaces where the number of lines and transfer rate were adapted to the specific wireless standard for which it was used. A characteristic for asynchronous communication with high-speed data bursts is the uncertainties in timing, i.e., no exact timing relation exists between the data samples of the burst and the data samples at the RF transceiver, However, wireless standards require a very accurate timing on the air interface in order to effectively manage access to the spectrum.
In a WiMAX system for example, the base station may advise the terminal to precisely align the transmit signal with a resolution as precise as 90 ns. In order to guarantee such an accurate timing, the latency of the datapath between the BB and the RF transceiver must be fixed. If control and I/Q data share the same serial interface, multiplexing is required and buffers are required on both sides of the interface, and the rate of that interface must be higher than the sum between the I/Q data rate and the control data rate. If the multiplexing is not fixed, but based on prioritization, typically some jitter will be introduced.
Moreover, keeping a precise timing on the air interface can be difficult to reach when, depending on signal conditions, different filters with different group delays are dynamically switched into or out of the receive (RX) or transmit (TX) chain. In particular, the terminal may detect the presence of an impulsive noise source and decide to switch on an impulsive noise-canceling block that had been in bypass mode before. By enabling this block, latencies are thus increased. In another example, this may also be the case when using a standard like IEEE 802.11g, which allows the use of different modes having different fundamental sample rates. For example, it allows the processing of an IEEE 802.11b signal with a sample rate of an integer multiple of 11 MHz, while the Orthogonal Frequency Division Multiplexing (OFDM) modes introduced in IEEE 802.11g are processed with a different sampling rate of 20 MHz. The differing modes are selected based on the detected preamble. Depending on the selected mode, the BB may decide to switch on a sample rate converter to convert the received signal to the desired frequency. The sample rate converter may add to the overall latency of the signal. Thus, the BB only sees arriving samples irrespective of the actual sample rate of the signal, and needs to keep track of the time based on those samples while considering the switching time of the sample rate converter and the conversion rate. In such a configuration, the BB needs to keep track with the latencies introduced by bypassing or not different processing blocks, which can be merely cumbersome.